This invention is concerned with techniques for efficiently coupling the energy output from multiple lasers.
Laser beam combining has been a subject of great interest in the field of high power lasers. Combining the power from multiple lasers is a highly desirable feature for such applications as phased-array laser radar and optical communications. The generic problem involves how to coherently combine two laser beams which have the same central frequency but a time-varying relative phase difference. Combining such beams is generally very difficult because the combining system must exhibit a dynamic response to phase variations.
In phased-array lasers, beam combining imposes very stringent conditions on the relative phases of the individual lasers. None of the phased-array diode lasers made to date provide for the control of these relative phases. Rather, each laser selects its own phase so as to minimize its threshold current. Generally, this process leads to undesirable phase differences between the laser beams emitted from the diode array.
Two laser beams can, in principle, be combined using a simple beam splitter or a grating, provided that the phases of the beams can be adjusted and maintained, both spatially and temporally, at optimum values. For a variety of reasons, however, such phase adjustment and maintenance is an extremely difficult task, particularly when moderate or high power lasers are involved. Active monitoring in time of the laser phases over the entire beam profiles is necessary. This phase monitoring is conventionally used to trigger servo systems to stabilize the phases for optimum coupling.
When an intracavity beam splitter is used as the coupling mechanism for injection locking two or more lasers, the phase of each injected beam generally will not coincide with the phase of the corresponding resonant cavity mode over any appreciable time interval.
For these reasons, and in spite of substantial ongoing efforts, the beam combining problem has not been adequately solved to date.